Fixed confusion with sort_index and unsorted in _lists/bbox-filter.

I now like 'unsorted' more because it has no underscore and it says "how many
unsorted items do we have before the list is sorted by decreasing size".
If error is zero we can skip simplification.
Reviewed app-couch communication scheme.

_view/bbox-filter showed serious latency at some tests with 2k+ documents and
multiple geometry levels. I suspect that CouchDB lists can't handle large
values AND doc count. And I don't need to work with the complete geometry if I
only conduct a bbox-type-size filtering anyway, I merely need a brief summary
for this. So the geometry update should work way faster in two or more steps
like this:
- _lists/bbox-filter/bbox-size returns GeoJSON ids and document ids and types.
- _lists/bbox-filter/geojson returns GeoJSON near the desired detail level, and
  full documents. I will query this with the ids returned by bbox-size.
- row.value.GeoJSON is not an array anymore, because it looked stupid.